Methods of acquiring tomographic images of living bodies and the like include X-ray computed tomography (CT) using X-rays, ultrasound CT using ultrasound, NMR-CT employing nuclear magnetic resonance, proton CT using beams of particles such as protons, and so forth. It is known that living bodies are transmissive to light. Thus, optical CT using light for tomographic images of small animals has been proposed (for example, see Patent Reference 1: Japanese Patent Application Laid-Open (JP-A) No. 11-173976 and Patent Reference 2: JP-A No. 11-337476).
Light that is illuminated at a living body is scattered inside the living body, and the scattered light is emitted from the periphery of the living body. Optical CT detects light that is scattered and reflected in a living body and emitted from the periphery of the living body, acquires electronic signals, applies predetermined signal (image signal) processing to the respective electronic signals, and reconstitutes images from the obtained information. Thus, optical CT provides tomographic images of the living body.
In the field of pathological testing, a drug or the like containing a phosphor that emits light of a predetermined wavelength may be supplied into a living body and optical CT (hereinafter referred to as fluorescence CT) may be used when movements and distributions of the drug, concentration levels if the drug concentrates at particular locations, and the like are to be observed. That is, excitation light that excites the phosphor is illuminated onto the living body, light (fluorescence) that is emitted from the living body in response to this excitation light is detected, and two-dimensional tomographic images and a three-dimensional tomographic image of the living body are reconstituted. Hence, information on locations, quantities and the like of the phosphor or of a test drug, cells or the like containing the phosphor is obtained from the tomographic images.
In this fluorescence CT, the excitation light is illuminated at a point on the surface of the living body, and the scattered fluorescence emitted from the living body in consequence is detected at numerous points. The illumination position of the excitation light is altered and the process is repeated. Thus, data in the amount of the number of illumination points multiplied by the number of detection points is obtained. Relationships between these data items are established in accordance with distributions of the fluorescent material and scattering and absorption characteristics of light in the living body, and tomographic images may be reconstituted on the basis of these relationships.
When a concentration distribution of a phosphor is calculated in fluorescence CT in order to reconstitute a tomographic image, applying an inverse problem calculation to two light intensity distributions—the excitation light intensity distribution and the fluorescent light intensity distribution—has been proposed (for example, see Patent Reference 3: Description of U.S. Patent Application Publication Ser. No. 2007/0286468 and Non-patent Reference 1: S. R. Arridge, “Optical tomography in medical”, Inverse Problems 15 (1999) R41-93).
In order to apply this proposal, it is necessary to detect both the excitation light intensity distribution and the fluorescent light intensity distribution. Therefore, an apparatus configuration capable of handling respective wavelengths of the excitation light is needed, and the inverse problem calculation needs to be carried out for two systems in order to obtain the respective light intensity distributions. This inverse problem calculation imposes a larger calculation load than a forward problem calculation and requires more time to calculate.